A pedestrian died on a motorway after getting out of a broken-down car and being hit by another vehicle, it is believed.
Police were called at 1.20am on Saturday to reports of a collision on the A1(M) involving a pedestrian and two vehicles close to the slip road at junction eight for Stevenage.
Paramedics were also at the site of the crash but a man in his 30s died at the scene.
Hertfordshire Police said it believed the man may have been getting out of a broken-down silver Honda Civic that had stopped on the grass verge near the slip road.
Drivers of both the cars involved in the crash stayed at the scene - the force said they were helping with the investigation.
Detective Sergeant Ben Heath, from the Serious Collision Investigation Unit, said: 'Firstly, my thoughts go out to the man's loved ones at this sad time.
'We are appealing for witnesses, information and dash cam footage as we work to establish the full circumstances surrounding the collision.
'At this stage, we believe the man may have been exiting a broken-down silver Honda Civic that was stationary on the grass verge near to the slip road.
A pedestrian in his 30s died after being hit during a collision on the A1(M) near Stevenage
'If you were travelling in the area at the time and recall seeing the vehicle, or if you have any information that you think may assist with our enquiries, please get in touch.'
DS Heath said anyone with information could report it to police through the force's website or anonymously through the independent charity Crimestoppers.